In recent years, improvements in characteristics of power devices have been required from the viewpoint of energy saving. To meet the requirements, power devices using SiC (silicon carbide) in addition to conventional power devices using Si (silicon) have been regarded as promising next-generation high-breakdown-voltage low-loss power switching devices. Examples of the power devices include an MISFET (a field effect transistor having a metal-insulator-semiconductor structure; in which an example of the insulator is an oxide such as a silicon oxide, and an example of the MIS is an MOS), a Schottky diode, and the like.
As an example, a MOSFET using SiC employs a device structure pursuant to the conventional device structure of a MOSFET using Si. Because SiC is greater in bandgap than Si, the SiC-MOSFET is capable of operating at higher temperatures than the conventional Si-MOSFET which has been operated at a temperature less than 200° C.
Patent Documents 1 to 5 disclose techniques related to semiconductor devices using SiC.
Patent Document 1: Japanese Patent Application Laid-Open No. 2005-310902
Patent Document 2: Japanese Patent Application Laid-Open No. 9-22922 (1997)
Patent Document 3: Japanese Patent Application Laid-Open No. 2006-32456
Patent Document 4: Japanese Patent Application Laid-Open No. 2000-101099
Patent Document 5: Japanese Patent Application Laid-Open No. 2005-268430
Conventionally, Al (aluminum) or an Al-based material having Al as a chief component and including alloys of Al with Si, Cu (copper), Ti (titanium), Pd (palladium) and the like has been used as a metal material for interconnection in power devices. However, when the Al-based material is employed as the metal material for interconnection, operation at a high temperature exceeding 200° C. is prone to cause the metal material to react with an electrode connected to a semiconductor region in a semiconductor substrate or a silicon film and the like formed on the surface of the semiconductor substrate, and to cause oxidation of the surface of the metal material, thereby degrading the reliability of the device.
In view of the above-mentioned problem with the Al-based material, Patent Document 1 described above proposes the use of a Cu-based material as an interconnect metal in a SiC power device. However, the thermal expansion coefficient of Cu is 17×10−6 K−1, and this value is significantly different from those of semiconductor materials such as Si (having a thermal expansion coefficient of 4.2×10−6 K−1) and SiC (having a thermal expansion coefficient of 3.7×10−6 K−1). For this reason, the use of the Cu-based material as the metal material for interconnection in power devices employing Si and SiC causes a strain in the power devices during high-temperature operation to present the problem of the reliability of the devices.